One commonly used technique for planarizing the surface of a wafer is chemical mechanical polishing (CMP). In a CMP process a wafer, as may be held by a wafer carrier head, is pressed against a polishing pad in the presence of a polishing slurry, and relative motion (rotational, orbital, linear, or a combination of these) between the wafer and the polishing pad is initiated. The mechanical abrasion of the wafer surface combined with the chemical interaction of the slurry with the material on the wafer surface ideally produces a planar surface.
As shown in FIG. 1, some known carrier heads, such as a carrier head 8, may include a flexible membrane 10 that contacts the back or unpolished surface of a wafer 12. A pressure chamber 14 may be defined in a frame 16 of the carrier head and such a pressure chamber 14 may be disposed behind the flexible membrane 10. Pressure chamber 14 is pressurized by a suitable externally connected pressurizing equipment (not shown) by way of a pressurizing port 15 so that a desired amount of compression force can be applied through membrane 10 to the back surface of the wafer to force the wafer 12 into contact with the polishing pad with a controlled pressure.
The carrier head also generally includes a wear ring 18 (sometimes referred to in the art as a “retaining ring” or “edge ring” but hereinafter referred to without limitation as a “wear ring”) that may be attached to an outer periphery of frame 16 and to a corresponding portion of membrane 10 (e.g., by way of glue or mechanical attachments 21). The wear ring 18 includes a surface 19 in frictional engagement with a polishing pad 20 and is configured to support the carrier head and wafer relative to the polishing pad 20. Surface 19 of wear ring 18 can eventually experience some erosion as a function of usage. That is, the height dimension of wear ring 18 identified by line 22 will be reduced and this height reduction of wear ring 18 tends to deform the flexible membrane 10 and create a discontinuity in the pressure applied to the wafer (and to the polishing pad) at or near a gap 24 between the wafer edge and the edge of the wear ring. This may adversely affect the uniformity of the surface of the wafer being polished, particularly near the wafer edge, since the non-uniform compression near the wafer edge may cause uneven polishing at that region of the wafer.